Police believed the collision occurred when the rotator blades of a chopper that was landing and one that was taking off touched, Lt. Phlunte Riddle said.

The collision caused extensive damage to both aircraft, crumpling the tail and breaking the blades of one and causing the blade to fall off and other damage to the other.

The injured were taken to hospitals for evaluation after the 4 p.m. collision at a helipad in Altadena. Riddle said they included a pilot and an observer in each of the Bell OH-58A helicopters and two officers on the ground. One of the observers was a civilian.

The weather was drizzly and cloudy at the time of the accident.

FAA spokesman Allen Kenitzer said his agency and the National Transportation Safety Board will investigate.

The collision follows a much more serious crash in Atlanta this month where two officers in a police helicopter were killed when the aircraft hit wires atop a power pole.

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